1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to updating electronic paper displays.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast, and readability indoors and outdoors. Because these displays attempt to mimic the characteristics of paper, these displays are referred to as electronic paper displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper, bi-stable and electrophoretic displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveal that in general, EPDs require less power and have higher spatial resolution; but have the disadvantages of slower update rates, less accurate gray level control, and lower color resolution. Many electronic paper displays are currently only grayscale devices. Color devices are becoming available although often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
Electronic Paper Displays are typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. This allows EPDs to maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously and power is only needed to change from one state to another. However, some devices are stable at multiple states and thus support multiple gray levels without power consumption.
Electronic paper displays are controlled by applying a waveform or array of values to a pixel instead of just a single value like a typical LCD. Some controllers for driving the displays are configured like an indexed color-mapped display. The framebuffer of these electronic paper displays contains an index to the waveform used to update that pixel instead of the waveform itself.
While electronic paper displays have many benefits, a problem is that most EPD technologies require a relatively long time to update the image as compared with conventional CRT or LCD displays. A typical LCD takes approximately 5 milliseconds to change to the correct value, supporting frame rates of up to two hundred frames per second (the achievable frame rate is typically limited by the ability of the display driver electronics to modify all the pixels in the display). In contrast, many electronic paper displays, e.g. the E Ink displays, take on the order of three hundred to one thousand milliseconds to change a pixel value from white to black. While this update time is generally sufficient for the page turning needed by electronic books, it is problematic for interactive applications like pen tracking, user interfaces, and the display of video.
One type of EPD called a microencapsulated electrophoretic (MEP) display moves hundreds of particles through a viscous fluid to update a single pixel. The viscous fluid limits the movement of the particles when no electric field is applied and gives the EPD its property of being able to retain an image without power. This fluid also restricts the particle movement when an electric field is applied and causes the display to be very slow to update compared to other types of displays.
When displaying a video or animation, each pixel should ideally be at the desired reflectance for the duration of the video frame, i.e. until the next requested reflectance is received. However, every display exhibits some latency between the request for a particular reflectance and the time when that reflectance is achieved. If a video is running at 10 frames per second and the time required to change a pixel is 10 milliseconds, the pixel will display the correct reflectance for 90 milliseconds and the effect will be as desired. If it takes one hundred milliseconds to change the pixel, it will be time to change the pixel to another reflectance just as the pixel achieves the correct reflectance of the prior frame. Finally, if it takes two hundred milliseconds for the pixel to change, the pixel will never have the correct reflectance except in the circumstance where the pixel was very near the correct reflectance already, i.e. slowly changing imagery.
Further, in current electronic paper displays, all pixels must be updated simultaneously. In order to change the entire display, the previous display change must be complete. The waveform used to update the display is based on the prior value and that value is unknown if an update is interrupted.
It would therefore be highly desirable to produce an electronic paper display that overcomes the update speed and contrast constraints of current electronic paper display, thus allowing for faster, and more “real-time”-like update of bi-stable displays.